From my experience with auto posting plugins and cross-posting on wpForo.

My auto-posting plugins are from CodeRevolution.

I had to use custom development. Because 1) wpForo is checking if there is a logged user when auto posting, and through cron there is no logged user and 2) Each rule in the auto-posting plugin has to send the id of the desired subforum you want to auto-cross-post, through Post Custom Fields.